1. Field of the Invention
The invention pertains to image intensified television camera systems particularly with respect to pulse modulating the image intensifier tube to effect automatic light control.
2. Description of the Prior Art
Television camera systems are known in the art that include automatic light control. In such automatic light control systems the average light intensity level impinging on the face plate of the television camera is maintained at a substantially constant level irrespective of scene illumination. Such prior art automatic light control systems generally utilize a motor driven iris to compensate for changes in total scene illumination. Alternatively a variable neutral-density filter prior to the lens is rotated to control the level of scene brightness entering the camera. Such devices are mechanical in nature and require continuous adjustment during normal operation. Friction and wear limit the performance characteristics and life of such systems. Additionally, motor and brush noise may cause undesirable interference in the system. Additionally since the prior art systems function by changing the total incoming light level, a moving scene may produce smear during the frame time of the camera.
Low light level television camera systems are known in the art that utilize a light amplification or image intensification device between the lens and the camera. Such image intensification devices may be utilized to maintain a reasonably constant level of brightness on the television display despite varying levels of brightness from the scene. A problem arises when endeavoring to utilize such low light level television systems over a wide dynamic range of light input. Present day image intensifiers are sensitive to damage caused by light input overload. Excessive light causes high current conduction in the device resulting in overheating which causes device damage. Additionally although a number of present day image intensifiers are gateable, the networks required for gating the image intensifier utilizing microsecond pulses are large and require excessive power rendering such devices unusable in a gated mode in practical applications. A high voltage delay-line pulse generator is an example of such an undesirable gating device.